Wireless communication systems have been extremely popular for more than a decade. They allow users to communicate with each other while remaining geographically mobile. In addition, these systems allow communications to be in different modes, such as full-duplex voice, half-duplex voice, and data, as examples. These systems also employ a plurality of base transceiver stations (BTS) to wirelessly transmit and receive information to and from mobile units (MU).
Early wireless communication systems, such as Global System for Mobile Communications (GSM) based systems, were designed primarily for the communication of speech and small text messaging. As wireless customers increased their use of wireless communication systems, they demanded more services and faster transmission speeds. In response to these demands, subsequent wireless communication systems, such as Global Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE) based systems, were developed to provide higher data rates that allow other types of wireless services, such as multimedia messaging. These systems are based on time division multiple access (TDMA) modulation scheme.
Continuing the trend for higher data rates to provide higher speed services, other modulation schemes were developed to achieve improved data rates. For example, wireless communication systems that use code division multiplexing (CDM) modulation scheme, such as Wideband CDMA based systems, were developed to attain higher data transmission speeds. In addition, data speed enhancements to WCDMA based systems have also been made with the introduction of High-Speed Downlink Packet Access (HSDPA) and IP Wireless.
For example, an HSDPA-based wireless communication system may be able to offer a peak data rate up to about 10 mega bits per second (Mbps). In the forward link, data are sent to mobile units (MU) via 10 milliseconds frames. Each WCDMA frame consists of five HSDPA transmit time intervals (TTIs), each having a width of two (2) milliseconds. Data are also sent to mobile units (MU) using code division multiplexing (CDM) modulation, and in particular, using 16 codes, also known as orthogonal variable spreading factor (OVSF). One of the 16 OVSF codes is generally reserved for control and pilot channels for synchronization and channel estimation. Accordingly, there are typically 15 OVSF codes reserved for transmitting traffic data to mobile units (MU). The IP Wireless protocol similarly uses OVSF for transmitting traffic data to mobiles.
One drawback of the HSDPA and IP Wireless data transmitting scheme is that it uses CDM modulation scheme. CDM modulation is generally susceptible to intra-cell interference due to the fact that the orthogonality of the OSVF codes degrades when they are transmitted over the air. This has a tendency to lower the achievable data rates of the HSDPA and IP Wireless based system.